1. Field of the Invention
The present invention relates to an absorbent structure comprising a microbial polysaccharide and to the process of making the same. The invention more particularly concerns an absorbent structure comprising a microbial polysaccharide having an increased pore size and a reduced bulk density.
2. Description of the Related Art
Polysaccharides such as cellulose, starch and gums, are used for many purposes. For example, cellulose is used in the manufacture of absorbents, paper products, materials of construction, fillers for food products, wound dressings, filtration mediums and other products well known to those skilled in the art. Polysaccharides are synthesized not only by various plant systems, but also by microorganisms capable of forming polysaccharides as part of their life cycle. Polysaccharides produced by microorganisms are termed microbial polysaccharides and include a network of structural elements which are produced by the microorganisms. The microbial polysaccharide has pores which are defined by the spaces between the structural elements and may or may not be fibrous in nature. For example, some microbial polysaccharides, such as microbial cellulose, include a network of individual fibers and have pores which are defined by the spaces between the individual fibers. The fibers of such microbial polysaccharides are orders of magnitude finer than fibers of polysaccharides produced by plants. Microbial polysaccharides have a much greater surface area per unit volume compared to polysaccharides produced by plants. Such microbial polysaccharides are highly absorbent and very useful in the manufacture of many of the same products as more conventional polysaccharides produced by plants.
Synthesis of microbial polysaccharides generally occurs by inoculating a quantity of an aqueous culture medium containing the appropriate nutrients with a polysaccharide-producing microorganism and incubating the resulting culture under the proper conditions for a sufficient time to generate the microbial polysaccharide. The synthesis may occur in a static aerobic culture or a culture under agitation such as in a fermenter. The microbial polysaccharide produced in the culture medium generally exists in an aqueous medium. Such aqueous medium generally contains, in addition to water, nutrients, cells, cell debris, and other materials.
Many of the products, such as, for example, absorbent materials, that are manufactured from microbial polysaccharides require that at least a portion of the aqueous medium in which the microbial polysaccharides are produced be removed prior to use. The desired amount of the aqueous medium is removed to improve the absorbency and appearance of the microbial polysaccharides and the absorbent products manufactured from the microbial polysaccharides.
Many conventional methods for removing a portion of the aqueous medium from the microbial polysaccharide are known to those skilled in the art and include air-drying, oven-drying, solvent-drying, freeze-drying and the like. However, microbial polysaccharides produced by some of these conventional methods of removing the aqueous medium have pores, which are defined by the spaces between the structural elements of the microbial polysaccharide, which collapse due to the electrostrictive or surface tension forces resulting from the removal. The collapsed pores reduce the liquid transport and retention properties (absorbency) of the microbial polysaccharides. Thus, to maximize the absorbency of the microbial polysaccharide, it is desirable to maintain the open, fine pores of the microbial polysaccharide during the removal of at least a portion of the aqueous medium.